1. Field of the Invention
The present invention relates generally to a keyboard and more particularly, to a circuit switch for a keyboard.
2. Description of the Related Art
To prevent multiple keys of a keyboard from misjudgment while they are pushed to work at the same time, a capacitive circuit board is usually used to serve as the detecting circuit of the keys. For example, U.S. Pat. No. 3,921,167 disclosed a capacitive circuit board, which includes two boards having respective printed circuits and an insulated layer mounted between the two boards. Capacitance can be generated between the printed circuits of the two boards and when the two printed circuits are electrically connected, the electric state of the circuit board can be detected.
Taiwan Pat. No. M377824 disclosed a keyboard membrane circuit board, which includes corresponsive electrodes located at the first and third membranes. Each of the electrodes is connected with a capacitive electrode in series. A second membrane is mounted between the first and third membranes and includes a plurality of through holes. Each of the through holes is located at a position corresponding to one of the electrodes, so the electrodes can contact one another via the through holes for conduction while the second membrane can isolate the first and third membranes from each other.
However, in the process of manufacture of the aforesaid second membrane, burrs or irregular surfaces are subject to formation around the through holes to make the first and third thin membranes fail to keep consistent interval therebetween or make the interval of some area therebetween smaller or larger to further make the capacitance generated between the first and third membranes fail to remain stable and to be accurately applied to detection of actions of the keys. Besides, electrodes and circuits need to be disposed on the first and third membranes, so it is not easy to add or adjust the layouts of other sensitive circuits or grounded circuits. In this way, the circuit design becomes more complicated and the size of the whole circuit hardware cannot be effectively reduced.